Sprinkling city parks with recycled water may create a breeding ground for hard-to-treat microbes. In a survey of parks in seven Chinese cities, researchers found that parks irrigated with treated wastewater were awash in signs of drug-resistant germs.

Even after the recycled water is treated in a sewage plant, it may carry microbes, drug-resistance genes and antibiotics that had washed down the drain. Sprayed into the environment, that water can spread microbes that could cause difficult-to-treat infections, the researchers say.

Ecologist Yong-Guan Zhu of the Chinese Academy of Sciences in Beijing and colleagues found that parks using recycled water had up to 8,655 times as much microbial antibiotic resistance genes as those using fresh water. Most of the resistance genes make microbes immune to antibiotics commonly prescribed for people, the authors report July 24 in Environmental Science & Technology.

China uses high levels of antibiotics and may not have the same sewage treatment methods as the United States, says civil engineer Amy Pruden of Virginia Tech in Blacksburg. These results may represent “the worst-case scenario,” she says. However, the United States also has drug-resistant microbes in its recycled water, which is becoming popular for irrigation in drought-prone communities. More research is needed to understand the health risks, she says.